Process of making alumina using iron as a catalyst



PROCESS on MAKING ALUMINA USING. IRON AS. A. CATALYST George .L. Hervert, DownersGrove, andHerman S. Bloch, Slrokie, Ill., assignorsfl tov Universallqil Prodncts Cornpany, Des Plaines, 1th, a corporationof Delaware No Drawing. Application October-2.4, 195.5 SerialNo. $42,468:

9' Claims. oi; 23-143 This application: is a continuation-in-part: of; our copending application Serial No; 405,508,. new Patent No; 2,746,842, filed January 21-, 19.54.-

This invention, relates to the preparation of alumina and more specificallyto a method of preparing an alumina hydrosol by the interaction of Water and metallic aluminum.

Alumina, either as the hydrate oras anhydrous aluminum oxide, is Widely used in many phases of the chemical and petroleum industries. It has been employed in the petroleum industry, as acatalysttorhydrocarbon conversion processes,- as a support for catalytically active materials to be used inhydrocarbon conversion processes and as a dehydrating; agent. Itiszwidely used in other industries for the; same purposes. The activated forms, which are consideredwto bemerely various physicalmodiv fications of aluminum oxide, are especially known for their pronounced catalytic activity and adsorptive capacity. The: use of alumina. as.a re,fractoryisalso well known. Alumina in the form of corundum. has been found suitable for use inthe manufacture of certain typesof refractory and ceramic, materials- In other uses'alu mina is mixed or composited with other compoundsto.

produce a wide variety of substances with. useful.properties. 7

It is an. objectof our. inventiontoprovide anew process. for the production of alumina.

The usual. commercial method. of producing'alumina is.

by purifying ores inwhich. theoxide-is present. Another method is by the.precipit-ation of aluminum hydroxide from its salts. The preparation of, alumina as at present practiced entailsthe addition of a} basic, reagent to, a. solution of aluminumchloride hexahydrate. The resultant precipitate is. washedv and. filtered. to. remove undesirable. impurities. r I

When alumina is. preparedfromthe. commercially available. aluminum. chloride hexahydrate, the. precipitated. alumina requires extensive washing and filtering in order to remove the impurities includingexcess chloride. The present invention otters a. novel method of preparing alumina which eliminates the need for Washing and filtration and thereby reduces the time and expenses hereinbefore entailed in purifying the alumina.

We have now discovered and our invention broadly comprises. an improved method of preparing, alumina by reacting aluminum with water under; specific conditions and in a specific manner. Hydrogen in a very; pure state is produced as a byproduct of" this. invention.

In one embodiment the present invention relates to a process for producing an alumina sol which comprises commingling aluminum containing less than 100 parts per million of iron (based on the aluminum) and an aqueous solution of an acid which reactable with alu-' minum to form hydrogeu rea'cting said solution and said aluminum until at least 25% of the hydrogen which is stoichiornetrically equivalent to the amount of acid initially present is evolved-,f subsequeh'tly adding aluminum containing from about 10ft to about 2500 parts per mil States ate" t ice lion of iron (based on. the aluminum), effecting; further reaction andrecovering alumina fromthe. reaction mix; ture, said acid being in amountof atleast. 0.13 equivalent of acid anion per equivalent ofaluminum.

In another embodiment the present invention relates to a process for producing an alumina. sol whichcom,- prises commingling aluminum containing less than. parts per million of iron (based on. the aluminum) and an aqueous solution of an inorganic acid whichis re actable with aluminum to form hydrogen, reaetingisaid solution and said aluminum until at least!25.%.. of thehy drogen which is stoichiometrically equivalent to the amount of acid initially present is evolved, subsequently adding a promoter comprising mercury and. aluminum charge containing from about 100. toabout 2500 parts per million of iron (based on the aluminum), effecting further reaction in the presence. of. said promoter and said aluminum, and recovering alumina from the reac; tion mixture, said acid being in amount of] at least.O.l.3 equivalentof'acid anion per equivalent of total aluminum. Inra specific embodiment the present invention relates. to a methodiof producing an alumina sol which comprises commingling aluminum containing less than IO Opa rts per million of iron (based on the aluminum) andfin aqueous solution of a mineral acidwhich is reactable: with aluminum to form hydrogen, at'a temperature of from about 30 F. to about 705 C., at a; pressure suf: ficient to maintain at least a portion of the water in thel liquid phase, reacting said solution and. said aluminum until atleast 25% of the hydrogen which is stoichio-. metrically equivalent to the amount of acid initially pres; cut is, evolved, subsequently adding" aluminum. conta' -f ing from about 100 to about 2500' parts prmillion of iron (based on the aluminum) and a promoter comprising mercury in amount of from about 0.05% to about. 50% by weight of the total aluminum, eifecting fu'rther reaction in' the presence of said promoter and said aluminum, and recovering alumina from' the reaction mix-i ture, said acid being in amount of atleast 0.13 equivalent of acid anion per equivalent of total aluminum.

Our inventionis basedon the discovery that'ironpromotes v the reaction between aluminum and water. The iron may be incorporated in the aluminum in any'suit able manner. A preferred method, however, is to-addi ironto molten aluminum, or'to-use aluminurn from-which iron. initiallyv present in the-aluminum source has not been completely removed. This method is prefered since the aluminum-iron mixture prepared by this method produces best results When used to prepare alumina. The ironmay be added to the aluminum. before or after it is molten. For. example, iron dust particles m'ay be contactedwithsolid aluminum and then the aluminum heated to above the melting point, or in another method the, aluminum. is already molten and the iron dust' or. iron-1 filings are added. to the melt. The mixture is a ho; mogeneous. mixture. when: this method of preparation. is: used; however, it. is. not definitely knownhow the; aluminum and iron. are associated in the mixture. There" may be some strong physical. or chemical. bonds. existing; in the solid mixture or the iron and aluminum maytbescompletely disassociated. Whatever. the physicalmakeuphovvever, the. presence of iron in: the. aluminum QOIlr' siderably speeds the aluminum-waterreaction. Iron ofxe.

ide or other iron compoundsl may be used instead.

metallic iron, since under the conditions .used' such mag terials are reduced by the molten aluminum metal.

We havealso found that the amount of iron used: promoter appears to be rather critical especially in the" to re? The acidic aqueous solution may be achieved by the addition of mineral acids and/or organic acids. These acids are normally added in only small amounts when it is desired to make alumina gel, and in somewhat larger but below stoichiometric amounts when a sol is being pre pared. For example. to make an alumina so] in the presence of hydrochloric acid, an amount of acid above about 0.4 mol per mol of aluminum, and pre erably about 0.65 mol per mol of aluminum, may be used. as compared with the stoichiometric amount of three mols. To make a gel,

amounts of acid one-tenth as large as those used for sol formation. or even less, may be used.

For purposes o alumina gel formation suitable mineral acids comprise the mono-basic acids, hvdrogen fluoride hydrogen chloride. hydrogen iodide. hydrogen bromide nitric acid. etc.; the bi valent acids, sulfuric acid. etc.: and the tr1-v alent acids, phosphoric acid. etc. Suitable orgame acids are the poly-basic acids such as for example oxalic acid. malonic acid. succinic acid, m leic acid phthalicacid. tartaric acid. citric acid. etc. These poly basic acids result in the desirable gel formation whereas the lower, water soluble mono-basic fatty acids do not. For purposes of el formation, the acid r mixture thereof must be selected in amounts so th t the acid anions to aluminum ratio be in terms of stoichi metric equivalents, as herein set forth. The mono-basic inor anic acids produce gels when the ratio of m no-valent acid anion is in amounts below 0.13 equivalent of acid anion per equiv alent of aluminum, while the bi-valent inorg nic acid anion is m amounts such that the ratio is below about 0.5. The polv-valent acid anions produce gels with a hi her ratio of bi-valent acidic anion of below 1.0 eouivalent of acid anion per eouivalent of aluminum and hi her; however. ratios below about 1.0 are preferred since ratios greater than 1.0 involve the use of excess free acid anion. For example. sulfuric acid forms gels with SO =/Al+++ ratios of below about 0.5 equivalent of acid anion per equivalent of aluminum while the corresponding maximum for Cl"/Al+++ is about 0.13. For gel formation, therefore. the acid is selected rom the group consistin of inorganic acids. oly-basic or polv-valent organic acids and ac d-acting salts in amount so that the mono-valent acid an on is in amount below about 0.13 e uivalent of acid an n oer e uivalent of aluminum, the bialent in rganic acid anion is in amount below about 0.50. the bi-valent organic acid ani n is in amount below about 1.0 and the amount of tri-valent acid anion is in amount below about 1.0. The bi-valent organic acid anions and higher valent or anic and inorg nic acid anions form gels above the 0.50 ratio of the di-basic inor anic acids with els being formed using ratios as hi h as 5.0. however, rati s much above 1.0 are not preferred since ratios above 1.0 involve the use of excess free acid. Ratios be ow about 0.01 of any of the acid anions do not effectively catalyze the desired reaction.

' For purposes of sol ormation suitable in r anic or mineral acids comprise hvdrogen chloride. hydrogen iodide, hydrogen bromide. nitric acid, sulfuric acid. etc., or mixture thereof. the acid being such that its aluminum salt is water-soluble. The amount of acid or mixtures thereof must be such that the anion/aluminum ratio be in terms of equivalents of at least 0.13 and generally within the range of from about 0.13 to about 0.75. The monobasic acids produce sols even with the lower ratios within this range, while the bivalent acids require the higherv ratios within this range. For example, sulfuric acid forms sols with SO =/Al+++ ratios of above about 0.5 equivalent per equivalent While the corresponding minimum for Cl-/Al+++ is about 0.13. The acids which form soluble aluminum salts are those having mono-valent or bi-valent anions, and the limiting or lower ratio of anion to aluminum ratio for sol formation (in equivalents of acid anion per equivalent of aluminum) may be broadly given by the formula,

where n=the valence of the anion (1 or 2). For monovalent anions, R=0.13; for,bi-valent, R=0.52 or about 0.5. Amounts below this range while possessing sufficient catalytic activity tend to produce alumina gels. The ratio of equivalents is determined with the total amount of aluminum, that is the aluminum having an iron concentration below 100 weight parts per million which is in the original reaction mixture plus the aluminum having an iron concentration of at least 101 weight parts per million which is added to the reaction mixture.

Using different concentrations of acids, therefore,'produces alumina either as a gel, a sol or crystals of alumina. At temperatures above about400 F. crystals of alumina are usually produced. The temperature range in which a fluid hydrogel or hydrosol is produced, therefore, is from about F. to about 400 F. although alumina may be produced within the broader range of from about 30 F. to about 705 F.

Hydrogen is evolved from the aqueous solution of the acid when contacted with solid aluminum, and the hydrogen may be produced by the reaction of the acid with the aluminum as well as by reaction of the aluminum with water. When the aluminum contains at least one hundred parts per million of iron (based on the aluminum) we have found that the reaction proceeds at a vigorous rate which may become quite violent and, therefore, the substances must be mixed slowly or periodically as the reaction proceeds. The rate of reaction depends upon the amount of acid and the amount of iron present in the aluminum. We have now discovered that when the acid is added to aluminum containing less than one hundred parts per million of iron (based on the aluminum), the reaction is much more easily controlled. Our invention, therefore, incorporates the steps of mixing the acid with aluminum containing less than one hundred parts per million of iron and after the reaction has proceeded for a time, and the reaction rate slows down aluminum containing from 100 to about 2500 parts per million of iron is added to the reaction mixture.

Hydrogen is evolved from the aqueous solution of the acid containing the solid aluminum of less than one hundred parts of iron per million, and, as hereinbefore mentioned, the hydrogen may be produced by the reaction of the acid with the aluminum and by the reaction of the aluminum with water. When the aluminum containing less than one hundred parts per million of iron and acidic aqueous solution are commingled, hydrogen is evolved and we have discovered that the benefits of the present invention, which are hereinbefore mentioned, may be most effectively utilized when the aluminum containing from 100 to about 2500 parts per million of iron is added after about 25% and before 95% and preferably after 50% of the hydrogen which is stoichiometrically equivalent to the amount of acid initially present is evolved. For example, if one mol of sulfuric acid is present in the aqueous solution, of the hydrogen which is stoichiometrically equivalent to the amount of acid is 0.75 mol of hydrogen; and, likewise if one mol of hydrochloric acid is present in the aqueous solution, 75 of the hydrogen which is stoichiometrically equivalent to this amount of acid is 0.375 mol of hydrogen. If aluminum containing at least one hundred parts per million of iron is added to the reaction solution before this amount of hydrogen is evolved the reaction often becomes violent and, also when a mercury promoter is used more mercury n arest promoter is ultimately-necessary to complete the reaction in a specified time than when adding the high iron content aluminum after this amount of hydrogen is evolved.

It is also preferred to add a mercury promoter, that is acatalyst comprising mercuryto the reaction solution When at least 25% and preferably at least 50% of the hydrogen which is stoichiometrically equivalent to the amount of acid initially present is evolved by the reaction of aluminum containing less than one hundred parts of iron per-million with the acidic solution. The mercury'promoter' and the aluminum containing at least one hundred parts of iron per million may be added simultaneously "or may *be added at different times; "however, neither one should be added before at least 25%, and preferably at least 50% of the hydrogen is evolved or otherwise the reaction becomes violent and readily gets out of control. The mercury promoter also acts as an accelerant to speed the reaction of aluminum with water in order to form the desired alumina. The acid also acts as an accelerant; however, acid anion is necessary in order to prepare an alumina sol. We have found that the mercury compound reacts with the aluminum and is reduced to metallic mercury which then appears to amalgamate with the aluminum. There is a difference in the catalytic effect of the various compounds of mercury and it. seems likely that the distribution of the mercury amalgam centers which are probably cathodic is an irnportant feature in determining the reaction velocity. However, we do not intend our invention to be limited by this theory. Acomparison between the speed of the reaction when metallic mercury is used as a promoter and a proportional amount of a mercury compound shows a definite advantage for the mercury compound although metallic mercury is itself quite effective.

Since the mercury promoter acts as an accelerant or a catalyst, it is preferably used in very low concentrations. Anyamount of a promoter used as an accelerant or catal-ystherein mentioned will be a catalytic amount of mercury or referred to as a catalytic amount. As hereinbefore mentioned the amount of the mercury promoter will usually be within the range of from about 0.05 to about 50% by weight of the aluminum. Concentrations of mercury promoter below this range do not effectively catalyze the reaction, and concentrations above this range often produce undesirable results, for example, the aluminum surface may be so completely amalgamated as to reduce the anodic area sufficiently to decrease the reaction velocity.

The mercury promoter is selected from the following group and may be a mixture of two or more of these mercury promoters: mercury, mercuric acetate, mercurous acetate, mercuric bromate, mercurous bromate, mercuricbrornide, mercurous bromide, mercuric bromide iod-ide, mercurous carbonate, mercuric chlorate, mercuric chloride, mercurous chloride, mercuric fluoride, mercurous fluoride, mercuric iodide, mercurous iodide, mercuric nitrate, mercurous nitrate, mercuric oxalate, mercuric oxide, mercurous oxide, mercuric sulfate, mercurous sulfate, etc., as well as mercury-nitrogen compounds such as ammono-basic mercuric bromide, ammonobasic mercuric chloride, etc. Almost any other mercurycontaining compound may be used and as herein mentioned it may be a mercuric salt, a mercurous salt either organic or inorganic, an oxide of mercury, or a complex of mercury compounds.

Drying the alumina at various temperatures produces alumina in various modifications. Drying within the temperature range of from about 30 F. to about 400 F. produces mainly Gibbsite alumina. An analysis of the alumina 'dried at 400 F. shows that the product is chiefly Gibbsite, however, small amounts of Bohmite, a modification of hydrated alumina, are evidenced. As the temperature of the drying is increased, the percent of Bohmite in the product is accordingly increased and at a tembe .;liqu'efied by pressure alone.

Vii

\SU perature of approximately 650 Frtheproduct afterthe drying is analyzed as a gained entirely Bohmite.

The exact role of the iron alsois not known. It 'appears that it is the free iron which is the promoter; how.- ever, the iron and aluininummay form a compound which is the actual promoter. Alternatively, the iron may be some means solubilize the surface coating of alumina that is formed by the reaction of aluminum with water causing the alumina to enterinto solution and expose more aluminum surface. The-ironactivatormay function to increase the electrochemical reaction by modifying the aluminum to a more'easily 'dissociatable structure or by suppressing the amount 'oflpolarization. For example, the iron may cause some internal or inter-granular stresses within the aluminum structure thereby rendering the alu-. minum more chemically reactive,

The iron is preferably used when mercury and/or a mercury compound is;also 'used as a catalyst to accelerate the reaction between the aluminum and water. The iron in such a reaction, that is one in which the-reactionis conducted in the presence of a mercury promoter, may have an effect upon the mercury, or the iron may aid in effecting the amalgamation ofthe aluminum with the-mercury. We do not intend to be limited to any of these theories, however, since, as hereinbefore mentioned, the exact role of the iron is not completely known, However, its use greatly affects the speed of the reaction betweenaluminum and water to form'alumina.

The degree of subdivision of'the"aluminum is another factor determining the rate of the reaction. The smaller the size of the particles, the greater the surface area of aluminum exposed to the water for reaction; a powdered aluminum, if not overly oxidized,-is I'l1616fOI''iXCllDl. Granulated or pelletedaluminum, or aluminum inribbon form is also suitable; however, the larger the particle size of the aluminum metal charge, the longer the time required for complete reaction. In general, pel'licles of not more than about aninch in greatest dimension are satisfactory, although those of 'less than about one-half inch average size are preferred. Aluminum :pel'lets prepared by dropping molten aluminum into water have proven to be yery satisfactory for producing alumina by the process of our invention.

The epsilon-alumina which forms rapidly on aluminum surfaces and acts as a coating which normally passivates aluminum does not effectively inhibit reaction under the conditions herein disclosed.

One embodiment of the present invention comprises agitating the reaction mixture sufliciently so that the reaction to produce alumina proceeds at a desirable rate. The reaction velocity is dependent upon the "teriiperatur'e of the reactants, the degree of's'ubdivisio'n of the "aluminum, and to a limited extent the concentration or amount of iron and of mercury, and the degree of agitation given the mixture. Thus a reaction that proceeds slowly at a temperature of 212 F. with only a mild agitation or shaking of the mixture will proceed very rapidly 'if the mixture is vigorously agitated. At a tempe'ratu'rle of 572 F., on the other hand, the reaction proceeds "relatively rapidly even withra mild'degree of external agitation. However, if the mixture is subjected to vigorous agitation, the time "necessary for complete reaction is sub stantially decreased. 7

It is a desirable feature of the present invention that liquid water be present, and it is thus necessary when temperatures above the boiling point are employed to effect thereaction under "sufficient-pressure 'to maintain a liquid phase of water. The critical temperature of water is 705.2" F; the definition' of the critical temperature being that temperature above which a gas cannot It is "desirable muse liquid water since it is much "easierfto =have -efficient mixing between the aluminum and wa-ter -accomplished if there is .a liquid :phase.

The use of iron appears to'ihave some -'effect 9n "theproperties of the alumina produced. For example, when iron is used in amounts of at least one hundred weight parts per million weight parts of aluminum, and the conditions of reaction are such that the alumina is formed as a slurry, the product slurry has ahigher filtration rate than when iron is not present. Further, we have found that the resulting alumina "powder is capable of being processed into pills of much greater strength than normally obtained with alumina prepared when the amount of iron is below one hundred weight parts per million.

The reaction of the aluminum with the acidic solution with and without the addition ofthe mercury promoter may be effected in anysuitable type of equipment wherein the reactants are subjected to agitation and preferably to vigorous stirring. The operation maybe carried out in continuous or batch-wisefashion. When temperatures above the normal boiling pointof water are employed, and the reaction is performed-with water in the liquid phase, it is of course necessary'that the reaction vessel be capable of withstanding'pressures sufficient to maintain a liquid phase of water. In small scale production of alumina by this'process a rotating pressure autoclave is satisfactory. When the temperatures employed are at or below the boiling point of water, the reaction may be effected in ordinary open equipment in which a means is provided for vigorous stirring, agitation or circulation of the reactants. It is,however, necessary that the process equipment be constructed of such material that it is not affected by water or aluminum and/or the promoters used so that undesirable elements are not introduced into the desired alumina product; however, if the presence of these foreign substances is not objectionable, the above precautions need not be adhered to. Hydrogen is produced by the reaction of the aluminum with the solution and a means of venting must be provided if the pressure build-up caused by the production of this hydrogen is to be avoided. If the equipment will withstand this additional pressure, however, it is not necessary to vent the hydrogen continuously. The amount of hydrogen evolved may be measured by the pressure build-up on the system and/or the amount of hydrogen vented may be passed through a gas meter and the amount evolved measured in this manner.

The following examples are given to further illustrate the novelty and utility of the present invention but not with the intention of unduly limiting the same.

Example I Eighteen grams of aluminum chips approximately i f wide, A" long, and ,4 thick, having five Weight parts per million of iron dissolved therein were utilized in this test. Distilled water (five hundred grams) and 0.4 gram of mercuric chloride were first placed in a two liter Pyrex flask equipped with a six-bladed Pyrex stirrer and heated. When the temperature reached 100 C. the aluminum chips were added. The hydrogen gas evolved was used to measure the amount of aluminum reacted and the rate of reaction. The stirrer was rotated at 1500 R. P. M. The maximum rate of reaction occurred near the beginning of the run and was 0.60 gram of aluminum reacted per hour per gram of unreacted aluminum. 14.3 hours were required to react 99.5% of the aluminum charge.

Example I! The experiment was repeated, this time using aluminum which had eight hundred parts per million of iron dissolved therein. The maximum velocity of reaction was 5.59 grams of aluminum per hour per gram of unreacted aluminum and in only 1.5 hours, 98.7% of the aluminum 'chargehad reacted.

- Examples I and II illustrate the accelerating eifect of several hundred parts of iron per million in the reaction between aluminum and water-to form alumina.

Example III Aluminum chips of approximately to wide, A" long and slightly more than A thick are used to prepare alumina. The aluminum contains 400 parts per million of iron. 23.5 grams of these aluminum chips are placed in a two liter creased Pyrex fiask equipped with two water condensers and a six-bladed Pyrex stirrer. The

stirrer speed is controlled at 1500 R. P. M. 47.5 mlrof reagent grade hydrochloric acid and 120.2 grams of (11S- tilled water are placed in the fiask and brought up to approximately 212 F. before adding the aluminum. temperature of approximately 212 F. is maintained in the flask by an electric heater. The hydrogen evolved by the reaction of the aluminum is passed through a wet test meter and measured. When the aluminum pellets are added the reaction is extremely vigorous and is dilficult to keep under control. I

The same run is repeated, however, in this case 8 5 grams of aluminum chips containing four parts per million of iron are first added to the reaction flask. The reaction proceeds much slower.

47.5 ml. of reagent grade HCl with a sp. gr. of 1.1851 contains 20.7 grams of HCl or 0.00125 lb. mols of HCl. The stoichiometric equivalent of hydrogen of this amount of acid is 0.00125/2 359 or 0.2245 cubic feet of hydrogen at standard conditions. 50 percent of this amount is 0.ll23 cubic feet. After approximately 0.15 cubic feet of hydrogen is evolved 0.1 gram of mercury and 15.0 grams of aluminum containing 330 weight parts per million of iron are added to the reaction mixture. The reaction velocity immediately accelerates, but can still be readily kept under control. Therefore, by adding the aluminum containing 330 Weight parts per million of iron after about two-thirds of the hydrogen equivalent to the original acid is evolved, a continuous high reaction rate is maintained Without the difiiculties attended on excessive reaction rates in the early stages, such as occur when both acid and a high iron content aluminum are present.

The reaction product is a clear water-white alumina $01 which may be readily gelled and dried to a hard, highly absorptive alumina particularly well adapted to use as a catalyst support.

We claim as our invention:

1. A process for producing alumina which comprises commingling aluminum containing less than parts per million of iron (based on the aluminum) and an aqueous solution of an acid which is reactable with aluminum to form hydrogen, reacting said solution and said aluminum until at least 25% of the hydrogen which is stoichiometrically equivalent to the amount of acid initially present is evolved, subsequently adding aluminum containing from about 100 to about 2500 parts per million of iron (based on the aluminum), effecting further reaction, and recovering alumina from the reaction mixture.

2. A process for producing an alumina sol which comprises commingling aluminum containing less than 100 part per million of iron (based on the aluminum) and an aqueous solution of an acid which is reactable with aluminum to form hydrogen, reacting said solution and said aluminum until at least 25% of the hydrogen which is stoichiometrically equivalent to the amount of acid initially present is evolved, subsequently adding aluminum containing from about 100 to about 2500 parts per million of iron (based on the aluminum), effecting further reaction, and recovering alumina from the reaction mixture, said acid being in amount of at least 0.13 equivalents of acid anion per equivalent of aluminum.

3. The method of claim 2 further characterized in that said acid is in amount of from about 0.13 to about 0.75 equivalent of acid anion per equivalent of total aluminum.

4. The method of claim 2 further characterized in that the reaction is effected at a temperature within the range of from about 30 F. to about 400 F.

5. A process for producing an alumina sol which comprises commingling aluminum containing less than 100 parts per million of iron (based on thealuminum) and an aqueous solution of an inorganic acid which is reactable with aluminum to form hydrogen and which forms water-soluble aluminum salts, reacting said solution and said aluminum until at least 25% of the hydrogen which is stoichiometrically equivalent to the amount of acid initially present is evolved, subsequently adding aluminum containing from about 100 to about 2500 parts per million of iron (based on the aluminum), and a promoter comprising mercury, elfecting further reaction in the presence of said promoter and said aluminum, and recovering alumina from the reaction mixture, said acid being in amount of at least 0.13 equivalent of acid anion per equivalent of total aluminum.

6. The method of claim further characterized in that said promoter is metallic mercury.

7. The method of claim 5 further characterized in that said promoter is a mercury salt.

8. The method of claim 5 further characterized in that said promoter is an oxide of mercury.

9. A method of producing an alumina sol which comprises commingling aluminum containing less than 100 parts per million of iron (based on the aluminum) and an aqueous solution of a mineral acid which is reactable with aluminum to form hydrogen, at a temperature of from about F. to about 705 F., at a pressure sufiicient to maintain at least a portion of the water in the liquid phase, reacting said solution and said aluminum until at least 25% of the hydrogen which is stoichiometrically equivalent to the amount of acid initially present is evolved, subsequently adding aluminum containing from about 100 to about 2500 parts per million of iron (based on the aluminum) and a promoter comprising mercury in amount of from about 0.05% to about by weight of the total aluminum, effecting further reaction in the presence of said promoter and said aluminum, and recovering alumina from the reaction mixture, said acid being in amount of at least 0.13 equivalents of acid anion per equivalent of total aluminum.

References Cited in the file of this patent UNITED STATES PATENTS 2,746,842 Bloch et a1 May 2, 1956 

1. A PROCESS FOR PRODUCING ALUMINA WHICH COMPRISES COMMINGLING ALUMINUM CONTAINING LESS THAN 100 PARTS PER MILLION OF IRON (BASED ON THE ALUMINUM) AND AN AQUEOUS SOLUTION OF AN ACID WHICH IS REACTABLE WITH ALUMINUM TO FORM HYDROGEN, REACTING SAID SOLUTION AND SAID ALUMINUM UNTIL AT LEAST 25% OF THE HYDROGEN WHICH IS STOICHIOMETRICALLY EQUIVALENT TO THE AMOUNT OF ACID INITIALLY PRESENT IS EVOLVED, SUBSEQUENTLY ADING ALUMINUM CONTAINING FROM ABOUT 100 TO ABOUT 2500 PARTS PER MILLION OF IRON (BASED ON THE ALUMINUM), EFFECTING FURTHER REACTION, AND RECOVERING ALUMINA FROM THE REACTION MIXTURE. 